Thinking about it, it's actually not that obvious how to support a malloc/free API. It seems like it would need to be deterministic to fit properly on the Web. But writing a spec for that is not easy since efficient malloc/frees are fairly complex and detailed. And once specced out, it could never be improved.
GC on the other hand already exists on the Web, and all the complexity is not noticeable (except for things like proper weak refs and finalizers, which is why those have not been added to the Web yet) so the spec is fairly simple and it allows constant optimization on the implementation side.
It's true that you need to ship malloc/free, but that can be really tiny too; https://github.com/fitzgen/wee_alloc is less than a kilobyte.
I don't know Rust that well, but what about unwinding and multithreading for example - don't you need to give those Rust features up if you don't want to ship any runtime code?
> It's true that you need to ship malloc/free, but that can be really tiny too
True, yeah. It's a tradeoff, though, tiny mallocs will be much slower than an optimized malloc (like dlmalloc) on real-world benchmarks.
malloc is several magnitudes simpler than GC though. Then there's the question when (if ever) wasm will have GC and what kind of GC will it be.
Not for everything, of course - if you use certain C#/Java/Kotlin features you'll need bundled runtime support. But if you avoid them, you don't.
For comparison, Rust can't emit tiny binaries if you use malloc/free, because it needs to bundle those. This is actually an area where Rust/C/C++ are at a disadvantage once wasm has GC, as GC will be "free" while malloc/free won't be, so many programs will be smaller as C#/Java/Kotlin rather than Rust/C/C++.